Magnetic recording media are widely used as a sound recording tape, video tape, computer tape, recording disc and the like. Ever-increasing demands have had to be met for increased magnetic recording density and shortened (higher frequency) recording wavelength. The recording system for these devices varies from an analogue system to a digital system. In response to the current demand for elevation of the magnetic recording density of the recording medium, a magnetic recording medium having a thin metal film, e.g., vapor-deposited, as the magnetic layer has been proposed. However, in view of the more facile productivity and practical reliability against corrosion or the like, a so-called coating type magnetic recording medium is nonetheless preferred. In a coating type magnetic recording medium, a dispersion of a ferromagnetic powder in a binder has been coated on the support. However, since a coated medium has a relatively low filling degree with respect to the magnetic substance, as compared with a medium having a thin metal film, the former is inferior to the latter with respect to the electromagnetic characteristic.
In any event, a widely used coating type magnetic recording medium comprises a dispersion of a ferromagnetic iron oxide, Co-modified ferromagnetic iron oxide powder, CrO.sub.2 powder or ferromagnetic alloy powder that has been coated on a non-magnetic support to form a magnetic layer thereon.
In order to improve the electromagnetic characteristic of such a coating type magnetic recording medium proposals have included, for example, improvement of the magnetic characteristic of the ferromagnetic powder added to the magnetic layer of the medium and smoothing of the surface of the medium. However, these proposed methods are not adequate to sufficiently elevate the magnetic recording density of the medium.
Recently, the recording wavelength for a coating type magnetic recording medium is being shortened concomitantly with elevation of the magnetic recording density of the medium. As a result, if the thickness of the coated magnetic layer is large, problems of self-demagnetization demagnetization loss in recording with a lowered output arise and thickness loss in reproduction are serious.
Therefore, in response, reduction in the thickness of the magnetic layer has been attempted. If, however, the thickness of the magnetic layer is reduced to about 2 .mu.m or less, the surface of the magnetic layer would often be influenced by the non-magnetic support so that the electromagnetic characteristic and drop-out of the medium would worsen. However, the influence of the rough surface of the support would be avoided if a thick non-magnetic undercoating layer is provided on the surface of the support and then a magnetic layer is coated over the undercoating layer as an upper layer, as proposed in JP-A 57-198536 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, that method still has a problem in that the head abrasion resistance and the head durability worsen. The problem is considered attributable to the fact that a thermosetting resin is used as a binder in the non-magnetic lower layer. As a consequence, the lower layer is hardened so that the friction between the magnetic layer and head, as well as the contact of the magnetic layer with other parts, are effected under no buffer condition. The magnetic recording layer having such a lower layer has poor flexibility.
In order to avoid the problem, the use of a non-hardening resin as a binder in the lower layer is conceivable.
In accordance with the conventional method where the lower layer is coated and dried and then the magnetic layer is coated thereover as an upper layer, however, the lower layer would be swollen by the organic solvent in the coating solution for the upper layer to cause turbulence of the coating solution for the upper layer. As a result, the surface property of the magnetic layer would thereby be worsened and the electromagnetic characteristic of the medium would thereby be lowered. However, in order to reduce the thickness of the magnetic layer, reduction of the amount of the magnetic coating solution for the upper layer, or reduction of the concentration of the magnetic coating solution by adding a large amount of a solvent thereto, may be envisioned. In the former case of reducing the amount of the coating solution for the magnetic coating layer, however, drying of the coated layer would start before allowing sufficient leveling time to pass after the coating .to cause another problem of coating defects of giving streaks or stamped patterns on the surface of the magnetic layer coated. As a result, the yield of the method would be extremely low.
On the other hand, in the latter case of using a magnetic coating solution having a low concentration, the coated film would be highly porous so that sufficient filling degree of a magnetic substance in the film could not be attained, and additionally, the strength of the coated film would be insufficient as the film is too porous. In short, both methods mentioned above have various unfavorable problems. Also, in the invention of JP-A-62-154225, poor yield is a serious problem left unresolved.
The present applicant has already proposed, as one means of overcoming the above problems, a method of simultaneous multi-coating system where a non-magnetic lower layer is provided and, while the lower layer is still wet, an upper magnetic layer containing a ferromagnetic powder is provided over the lower layer, as described in JP-A-63-191315 (corresponding to U.S. Pat. No. 4,963,433) and JP-A-63-187418 (corresponding to U.S. Pat. No. 4,863,793). This technique yields a magnetic recording medium with high producibility, wherein the medium is free from coating defects and has elevated productivity, improved electromagnetic characteristics such as output and C/N ratio and improved running durability.
In accordance with the conventional method, however, it is still difficult to satisfy the current demand for a high-recording density magnetic recording medium having a thin coated layer suited for extended use, in particular, for such a device having a thin magnetic layer having a thickness of 1 .mu.m or less. That is, if such a thin high-recording density magnetic recording medium is prepared by the conventional method, the medium has coating defects and another problem that both the electromagnetic characteristic and the running durability could not be satisfied well. In particular, when the running durability of a thin tape is desired to be increased, reduction of damage of the tape edge is necessary. However, the conventional method could not satisfy the demands.